In recent years, substantial research has been conducted into various catalytic methods for making biobased 1,2-propanediol (commercially known as propylene glycol).
A number of these methods are directed to manufacturing propylene glycol from glycerol, as a means of making productive use of the glycerol produced as a byproduct of making biodiesel, however, a variety of other feedstocks have been converted to propylene glycol as well.
Copper-containing catalysts have been extensively evaluated in many forms for use in these conversions. WO 2014/134733 to Dalal et al. is a recent example of work in the copper-containing catalysts in the field of converting glycerol to propylene glycol, and after reviewing a number of prior art methods involving both homogeneous and heterogeneous catalysts describes a process for the hydrogenolysis of glycerol to produce propylene glycol as the major product, which process comprises reacting the glycerol with hydrogen in the presence of a heterogeneous multicomponent catalyst based on Cu, Zn, Cr and Zr prepared by a co-precipitation method. The multicomponent catalyst was identified for further study after an initial screening of a number of catalysts in certain molar ratios, including Cu:Zn:Ni (3:2:2), Cu:Cr:Ni (3:1:2), Cu:Zn:Cr (3:2:1), Cu:Zn:Cr:Ni (3:2:1:2), Cu:Zn:Cr:Zr (3:4:1:3) and Cu:Zn:Cr:Zr (3:2:1:3).
Interestingly, while Dalal et al. reference prior publications by Chaminand et al. (Green Chemistry, 2004, vol. 6, pages 359-361) and Maris et al. (Journal of Catalysis, 2007, vol. 249, pp. 328-337) as support for Dalal's statement that “the Cu/ZnO based catalysts have been reported to give a high catalytic performance for the glycerol dehydroxylation reaction to propylene glycol under mild reaction conditions”, on page 359 of Chaminand et al., a CuO-ZnO catalyst was initially selected for evaluation because of its efficiency in the hydrogenolysis of sorbitol to deoxyhexitols, but was found to have low activity and low conversion in glycerol hydrogenolysis (though it was observed to have high selectivity to propylene glycol consistent with the earlier findings of Montassier et al. (Montassier et al., Bulletin de la Societé Chimique de France 1989, No. 2, pp. 148-155) with a Raney copper catalyst).
Balaraju et al., “Selective Hydrogenolysis of Glycerol to 1,2-Propanediol Over Cu—ZnO Catalysts”, Catal. Lett., vol. 126, pp. 119-124 (2008) report, however, “high conversion” with “highly selective” Cu—ZnO catalysts under certain conditions at a 50:50 weight ratio of copper to zinc and with small Cu and ZnO particles.
Copper-containing catalyst systems are addressed also in a series of patents assigned to BASF SE, see, e.g., U.S. Pat. Nos. 7,790,937, 8,252,962, 8,273,924 and 8,293,951 all to Henkelmann et al. In U.S. Pat. No. 8,293,951, after reviewing prior references employing various catalysts—Cr-activated copper or cobalt catalysts, nickel, copper-chromium-barium oxide, Raney copper, supported metal catalysts based on Cu, Pd and Rh, copper chromite, copper zinc oxide, copper aluminum oxide, copper silicon dioxide, platinum, cobalt/copper catalysts optionally containing manganese and/or molybdenum—a process is described employing at least three hydrogenation reactors in series with a heterogeneous copper catalyst. The copper catalyst is broadly described, and may additionally comprise at least one further element of “main group I, II, III, IV or V, of transition group I, II, IV, V, VI, VII or VIII and of the lanthanides (IUPAC Groups 1-15 and the lanthanides”, col. 18, lines 26-30, though Raney copper and copper alloy-containing catalysts are preferred, particularly those whose metal component consists of copper to an extent of at least 95%, especially to an extent of 99%, col. 18, lines 32-39. Specific combinations of copper with other metals, in oxidic form, reduced elemental form or a combination are also listed, with certain combinations indicated as preferred: Cu (preferred); Cu,Ti (preferred); Cu, Zr; Cu, Mn; Cu, Al (preferred); Cu, Ni, Mn; Cu, Al, at least one other from La (preferred), W, Mo, Mn, Zn (preferred), Ti, Zr, Sn, Ni, Co; Cu, Zn, Zr (preferred); Cu, Cr, Ca; Cu, Cr, C (preferred); and Cu, Al, Mn (preferred) and Zr if appropriate. While very many combinations of other metals are thus indicated in this patent or are mentioned as known from the prior art, this particular patent contains but a single example, using a catalyst composed of the mixed oxides of Cu, Al and La.